1. Field of the Invention
The present invention relates to an ignition coil mounting structure for an internal combustion engine, in particular to an ignition coil mounting structure for mounting an ignition coil for each engine cylinder, independently.
2. Description of the Prior Art
A low power loss ignition apparatus is known such that a relatively small-sized ignition coil is provided for each cylinder head and a high tension (voltage) generated by each ignition coil is directly supplied to each ignition plug, as disclosed in Japanese Published Unexamined Utility Model application Ser. No. 62-74173, for instance.
FIG. 1 shows this prior art ignition apparatus, which comprises an ignition plug 1, an ignition coil casing 2 extending from the ignition plug 1 toward the upward direction, and an ignition coil 3 housed within the coil casing 2. When this prior-art ignition apparatus as shown in FIG. 1 is mounted on a DOHC (double overhead camshaft) engine, for instance, the coil casing 2 is mounted vertically passing through a hole formed in a rocker cover 4 and a middle straight hollow space 7 formed in a cylinder head 5. Further, the lowermost end of the coil casing 2 is screwed into a threaded ignition plug hole formed at the middle of the cylinder head 5 and the uppermost end of the coil casing 2 is fixed to the middle of the rocker cover 4 with a nut 6, so that the coil casing 2 is mounted vertically on the cylinder head 5.
In the above-mentioned prior-art ignition apparatus, however, since the ignition plug 1 and the ignition coil 3 11 are formed integral with each other within the casing and further the ignition plug 1 is fixed to the cylinder head 5 by inserting an end 2a of the casing 2 into an ignition hole via screw thread, the height and angular positions of the ignition plug 1 are determined after the ignition coil has been fixed to the cylinder head 5. In other words, there exists a problem in that the height and angular positions of the ignition plug 1 are not determined accurately, so that the orientation of an electric connector provided on top of the ignition coil 3 changes according to each cylinder; that is, the electric connectors are not arranged uniformly for all the cylinders, thus deteriorating the productivity of ignition plug mounting process into the cylinders. In addition, in the prior-art, since the ignition plug 1 and the ignition coil 3 are formed integral with each other, there exists another problem in that both the ignition plug 1 and the ignition coil 3 must be removed together from the cylinder head 5 and then disassembled in the case of maintenance, thus complicating the ignition plug mounting work.
Further, since the middle hollow space 7 through which the coil casing 2 is housed communicates with two cam journals 8 and two cam chambers 9 for accommodating cam bearings on the cylinder head 5, lubricant supplied to these cam journals 8 flows downward from the cam chambers 9 into the middle hollow space 7 as shown by two curved arrows. Therefore, there exists a problem in that the coil casing 2 is contaminated by the lubricant, in particular when the coil casing 2 is fixed to or removed from the cylinder head 5, so that the lubricant tends to reach the ignition plug 1 and therefore the ignition reliability is deteriorated. In addition, when the lubricant flows into the coil casing 2, there exist a harmful influence upon the various elements such as ignition coils, insulating members, etc.